1. Field of the Invention
One embodiment of the present invention relates to a storage battery electrode, a manufacturing method thereof, a storage battery, and an electronic device.
Note that one embodiment of the present invention is not limited to the above technical field. One embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. One embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. Specific examples of the technical field of one embodiment of the present invention disclosed in this specification include a semiconductor device, a display device, a light-emitting device, a power storage device, a memory device, a method for driving any of them, and a method for manufacturing any of them.
2. Description of the Related Art
With the recent rapid spread of portable electronic devices such as mobile phones, smartphones, electronic book (e-book) readers, and portable game machines, secondary batteries for drive power sources have been increasingly required to be smaller and to have higher capacity. Nonaqueous secondary batteries typified by lithium-ion secondary batteries, which have advantages such as high energy density and high capacity, have been widely used as secondary batteries for portable electronic devices.
A lithium-ion secondary battery, which is one of nonaqueous secondary batteries and widely used due to its high energy density, includes a positive electrode including an active material such as lithium cobalt oxide (LiCoO2) or lithium iron phosphate (LiFePO4), a negative electrode including an active material such as graphite capable of reception and release of lithium ions, a nonaqueous electrolytic solution in which an electrolyte formed of a lithium salt such as LiBF4 or LiPF6 is dissolved in an organic solvent such as ethylene carbonate or diethyl carbonate, and the like. The lithium-ion secondary battery is charged and discharged in such a way that lithium ions in the secondary battery move between the positive electrode and the negative electrode through the nonaqueous electrolytic solution and are inserted into or extracted from the active materials of the positive electrode and the negative electrode.
A binder is mixed into the positive electrode or the negative electrode in order that active material particles can be bound to each other or an active material layer and a current collector can be bound. Since the binder is generally an organic high molecular compound such as polyvinylidene fluoride (PVdF) which has an insulating property, the electric conductivity of the binder is extremely low. Furthermore, the binder has no charge storage capability. Thus, as the ratio of the amount of the binder to the amount of the active material is increased, the output voltage of the secondary battery is decreased due to high inner resistance, and in addition, the proportion of the amount of the active material in the electrode is relatively decreased. As a result, discharge capacity of the secondary battery is decreased.
Hence, by mixture of a conductive additive such as acetylene black (AB) or graphite particles, the electric conductivity between active material particles or between an active material layer and a current collector can be improved. Thus, an active material layer with high electrical conductivity can be provided (see Patent Document 1).
An electrode including graphene as a conductive additive has been developed. Patent Documents 2 and 3 each disclose an electrode manufacturing method in which graphene oxide (GO) which is an oxidized derivative of graphene, an active material, and a binder are mixed and then the GO is reduced. By this manufacturing method, an active material layer having high electrical conductivity only with a small amount of the conductive additive can be provided.